1. Technical Field
The present invention relates generally to a coagulator in which a coagulant is mixed into raw water, and suspended solids in the raw water are settled in a coagulated manner for separation in a coagulation tank, and more particularly to a coagulator having a packing medium accumulation layer(s) within the coagulation tank.
2. Background Art
Coagulators are known as water treatment apparatuses for use in cases where river water or the like, as raw water, is subjected to water treatment to obtain city water or industrial water, or where drainage such as public sewage and factory waste water is treated to meet regulatory limits. Among them, frequently employed is a so-called upward flow type coagulator, in particular, one having a packing medium accumulation layer in the interior of the coagulation tank because, for example, it has higher coagulating and filtering efficiencies and is easy to operate.
In such an upward flow type coagulator, a coagulant is first mixed into raw water. Raw water bearing the coagulant is then passed through the interior of the coagulation tank with upward flow to coagulatively separate flocculated flocs. In particular, within the coagulation tank there is provided a packing medium accumulation layer consisting of small pieces of packing medium with a large void ratio, through which the raw water bearing the coagulant flows with upward flow to thereby effectively flocculate and settle the suspended solids in the raw water.
Referring now to FIGS. 5 and 6, description will be given of a construction and an operation method of the conventional upward flow type coagulator. FIG. 5 is a flow sheet showing the construction of the conventional upward flow type coagulator, and FIG. 6 is a schematic diagram showing the construction of a coagulating tank.
The conventional upward flow type coagulator designated generally at 10 comprises, as shown in FIG. 5, a raw water tank 12, a raw water pump 14 for pumping raw water from the raw water tank 12 to feed the water, a coagulant dosing facilities 16, a coagulant mixing tank 18 and a coagulation tank 20.
The coagulant dosing facilities 16 includes a turbidimeter 22 for measuring the turbidity of the raw water, a coagulant tank 24, and a coagulant pump 28 for injecting the coagulant from the coagulant tank 24 into a raw water supply pipe 26 downstream of the turbidimeter 22 so that a desired amount of coagulant is added into the raw water on the basis of measured value of the turbidimeter 22.
The coagulant mixing tank 18 is a container fitted with a stirrer 29, in which the raw water bearing the coagulant temporarily stays and is stirred by the stirrer 29 to rapidly mix the raw water and the coagulant. The raw water fully mixed with the coagulant is sent via an inflow pipe 30 to the coagulation tank 20.
The coagulation tank 20 is a tank in which coagulative flocs containing suspended solids which have been aggregated by the coagulant are flocculated and filtrated and separated. The coagulation tank 20 consists of a raw water inflow zone 32, a packing medium 34 and a water collection zone 36 which are partitioned in sequence from below as shown in FIG. 6.
The packing medium zone 34 is partitioned by outflow prevention plates 38 and 40 which are porous mesh-like or perforated like partition plates provided in an upper part and lower part of the zone. In the region therebetween are accommodated a plurality of small packing medium with a large void ratio, e.g., plastic small packing medium 42 with a relatively small specific gravity in the shape of shortened tube as shown in FIG. 7.
Upon the passage of the raw water, the packing medium 42 is accumulated below the upper outflow prevention plate 38 with the upward flow of the raw water to form a packing medium accumulation layer 44, whereas a water flow region 45 is formed in between the packing medium accumulation layer 44 and the lower outflow prevention plate 40, with no or little substances except the flowing water.
The water collection zone 36 is a region for collecting the treated water which has been treated through the packing medium zone 34, the zone 36 consisting of a water collecting part 46 disposed immediately above the outflow prevention plate 38 of the packing medium zone 34, a water collection trough 48 for collecting treated water overflowing from the upper end of the water collection part 46, and an outflow pipe 50 connected to the water collection trough 48 for sending the treated water to the treated water tank 52 (see FIG. 5).
The raw water leaving the coagulant mixing tank 18 flows via the inflow pipe 30 into the inflow zone 32. The inflow pipe 30 which extends to the middle of the inflow zone 32, and has at its tip a downward opening. An inverted umbrella shaped baffle plate 54 is provided under the opening of the inflow pipe 30 for changing the direction of the downward flowing raw water to upward direction. An alkaline agent injection pipe 56 is also connected to the inflow pipe 30 so as to inject an alkaline solution, if desired, for the control of the pH of the raw water.
In the lower part of the inflow zone 32, that is, below the baffle plate 54, there lies a hopper shaped sludge storage zone 58 for storing the sludge, to whose lowermost part is connected a sludge discharge pipe 60 for discharging the sludge.
Above the inflow zone 32 is disposed an air supply pipe 62 having a plurality of air nozzles for jetting air upward, so as to eject air fed by the air blower 64 to thereby stir and cleanse the packing medium 42 of the packing medium accumulation zone 34.
In the coagulation tank 20 the raw water bearing the coagulant first flows into the inflow zone 32. First settled and separated in the inflow zone 32 are relatively large flocs among flocs formed as a result of coagulation of the suspended solids in the raw water.
The raw water then flows into the packing medium zone 34, in which micro flocs remaining in the raw water come into contact with the packing medium 42 and adhere onto the external surface of the packing medium and onto the inner surface of the packing medium or is captured in the interstices between the packing medium 42 and is separated from water. On the other hand, the raw water flows through voids of the packing medium 42 or flows through between the packing medium 42, and is filtrated through the floc layer formed in the voids or between the packing medium, while simultaneously micro flocs in the raw water are captured by the floc layer.
The flocs which have adhered onto the packing medium 42 or have been captured between the packing medium grow gradually due to contact with the subsequent micro flocs or the like, resulting in an increase of the floc diameter. Then, accordingly as the flocs having a higher settlement velocity than the upward flow rate of the raw water become formed, those flocs are dislodged from the packing medium 42 due to the flow of raw water and further settle against the flow of the raw water and are retained in the sludge storage zone 58, and then discharged from the sludge discharge pipe 60.
In this manner, the suspended solids in the raw water are separated from the raw water and settled in the sludge storage zone 58 due to the coagulation function of the suspended solid flocs, filtration function thereof for the raw water, and separation and settlement function of the floc. On the other hand, the raw water flows as the treated water from the upper water collection zone 36 into treated water tank 52.
This upward flow type coagulator enables a high speed treatment to be performed since the density of the coagulated flocs which have become thickly agglomerated is high with correspondingly higher settlement speed. Accordingly, the facilities become compact, resulting in a smaller facility area, with reduced dosages of chemical agents (coagulants, etc.), and with easy treatment and disposal of the generated sludge.